Method, paper machine and base paper for the manufacture of lwc printing paper coated once

ABSTRACT

A paper machine for the manufacture of LWC printing paper which is coated once has a headbox ( 100 ), a gap former ( 200 ), a press section ( 300 ) having at least one extended nip press, a pre-dryer section ( 400 ) in which a web (W) is dried applying at least cylinder drying (R 1 -R 7 ), a precalender ( 500 ) in which the web (W) is pre-calendered, at least one portion which is formed of a coating station ( 600 ) and an after-dryer section ( 700 ) and in which the web (W) is coated on both sides applying a film coating method or a non-contact coating method and dried applying at least contact-free drying ( 710 ), an end calender ( 800 ) in which the web (W) is calendered, and a reel-up ( 900 ) in which the web (W) is reeled. The pre-calender ( 500 ) is a calender which is provided with at least one nip (NE 1 , NE 2 ) and in which both surfaces of the web are in contact with a calendering backup surface having a surface temperature of at least 200° C., advantageously at least 250° C., the total length of the nip or nips of the pre-calender is in a range of 15-600 mm, advantageously in a range of 30-600 mm, the linear load of each nip is in a range of 50-500 kN/m, advantageously in a range of 100-400 kN/m, and the moisture content of the web before the first nip of the pre-calender ( 500 ) is in a range of 5-20%, advantageously in a range of 6-15%.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a U.S. National Stage application of InternationalApplication No. PCT/FI02/00528, filed Jun. 17, 2002, and claims priorityon Finnish Application No. 20011291, Filed Jun. 18, 2001.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to a method for the manufacture of LWC printingpaper coated once, and to a paper machine for the manufacture of LWCprinting paper coated once, and to a base paper intended for LWCprinting paper.

FI patent application 991096 discloses a method and a paper machineline, in particular for the manufacture of fine paper. By fine paper ismeant uncoated or coated fine paper. The basis weight of uncoated finepaper is normally in a range of 40-230 g/m² and the basis weight ofcoated fine paper is in a range of 60-250 g/m². The proportion ofmechanical pulp in fine paper is generally below 10% and fillers areadded to the stock in an amount of about 15-30%. Recycled fibres canalso be used in the pulp. The paper machine line comprises a shortcirculation the stock volume of which has been minimized, a headbox, agap former, a press section comprising at least one extended nip press,a dryer section of which at least a portion is based on impingementdrying, a pre-calender, a two-side pre-coater and a dryer section afterthat, an on-line coating station/stations and an after-dryersection/sections mainly based on contact-free drying after the on-linecoating station/stations, an on-line calender in which the linear loadscan be regulated separately in each nip, and a reel-up. The pre-calendercan be a hard nip calender, a soft calender or an extended nip calender.Relatively low linear loads are used in the pre-calender, for example,below 80 kN/m. Pre-coating can be accomplished by means of a filmtransfer coating device marketed under the name SymSizer or OptiSizer.The function of pre-coating is to make the pores in the surfacestructure of the base paper smaller in a suitable manner in order thatthe actual surface coating shall remain on the surface and is notabsorbed into the structure of the paper. In coating it is possible touse a suitable coating station, for example, a blade coater, a coatingdevice of the jet or spray type, advantageously a coating devicemarketed under the name OptiCoat Jet. Here, the paper machine inquestion is thus a paper machine in which coating is accomplished twice.

FI patent 104100 discloses an integrated paper machine. The papermachine comprises, in the running direction of the web, a multi-layerheadbox, a gap former having at least one pre-press, a press sectionhaving at least one extended nip press, a pre-dryer section in which theweb is dried by a high-capacity drying unit, a dryer section comprisingat least one drying group that applies single-wire draw, and a surfacetreatment device for the web. The paper machine has a closed draw atleast to the end of the dryer section. The surface treatment device maybe formed of a pre-calender, which is provided between a drying cylinderand a roll.

FI patent application 981331 discloses a method and a paper machine forthe manufacture of paper. The method and the paper machine are mostsuitable for the manufacture of glossy and porous paper for color powderprinting. The paper machine comprises a headbox, a wire section, a presssection, a dryer section, a coating section, an after-dryer section, acalender, and a reel-up. The headbox and the wire section are designedsuch that paper is provided with a desired composition layering in thez-direction and that the calender is a calender device that maintains orat least substantially preserves the porosity of the paper web existingbefore calendering.

In this patent application, by LWC printing paper (Light Weight Coated)which is coated once is meant a printing paper the basis weight of whichis in a range of 35-70 g/m² and in which the basis weight of the coatingis in a range of 2-12 g/m²/side. Each side of the base paper is coatedonce. In the arrangements of the state of the art intended for themanufacture of this kind of printing paper, the base paper is subjectedto light pre-calendering after the dryer section before coating. Thepre-calendering according to the state of the art is carried out by aone-nip machine calender such that the thermo roll temperature is in arange of 60-100° C. and the linear load is in a range of 10-80 kN/m.Pre-calendering is performed in the state-of-the-art arrangements withlow linear loads in order not to lose too much bulk in pre-calendering.This kind of state-of-the-art light pre-calendering provides base paperhaving a PPS-s 10 smoothness in a range of 4-5.7 μm, Cobb-Unger oilabsorption in a range of 13-26 g/m² and bulk in a range of 1.7-1.9cm³/g.

Out of this kind of state-of-the-art base paper it is possible tomanufacture LWCO printing paper (Light Weight Coated Offset), i.e.lightly coated offset printing paper using a film coating method or anon-contact coating method. On the other hand, out of this kind ofstate-of-the-art base paper it is not possible to manufacture LWCRprinting paper (Light Weight Coated Rotogravure), i.e. lightly coatedrotogravure paper using a film coating method or a non-contact coatingmethod. This is due to the fact that the film coating method as well asthe non-contact coating methods do not cover the uneven surfacestructure of the base paper in single coating, but this uneven surfacestructure of the base paper is also visible in the surface structure ofthe coated paper. Therefore, LWCR printing paper manufactured out ofstate-of-the-art base paper must be made by a blade coating method,whereby the uneven surface structure of the base paper is covered incoating and the finished coated paper attains the surface propertiesrequired from LWCR printing paper. The blade coating method in turnrequires that the base paper contains at least 40-50% chemical pulp andthat there is hardly any recycled fibres in the base paper. Theabove-mentioned amount of chemical pulp provides the base paper with thestrength required in blade coating. The blade coating method limits thespeed of the paper machine in on-line coating to a value of about 1800m/min.

Since the film coating method is, by a rough estimate, about 5% moreefficient than the blade coating method and since the film coatingmethod does not impose limitations on the speed of the machine inon-line coating, it would be desirable to replace the blade coatingmethod with a film coating method or with a non-contact coating methodalso in the manufacture of LWCR printing paper. In order that LWCRprinting paper might be manufactured by a film coating method ornon-contact coating method, the PPS-s10 roughness of the base papershall be below 3.5 μm and its Cobb-Unger oil absorption shall be below15 g/m². A reduction in roughness and oil absorbency in turn requiresmore efficient pre-calendering, with the result that the base paper isdensified and some bulk is lost. The loss of bulk of the base paper hasbeen thought to also lead to a loss of bulk of equal magnitude in thecoated end product and thus, in the opinion of the person skilled in theart, the manufacture of LWCR printing paper has not been possible byusing a film coating method or a non-contact coating method.

SUMMARY OF THE INVENTION

However, in the invention it has been surprisingly found that the bulklost in reinforced pre-calendering does not necessarily lead to acorresponding loss of bulk in the end product. By means of the method inaccordance with the invention it is possible to manufacture LWC printingpaper which is coated once and the bulk of which is at least as good asthe bulk of LWC printing paper manufactured according to the state ofthe art.

The arrangement in accordance with the invention employs a film coatingmethod or a non-contact coating method and reinforced pre-calendering.The reinforced pre-calendering in accordance with the invention providesa base paper out of which it is possible to manufacture LWCR printingpaper using a film coating method or a non-contact coating method.

The reinforced pre-calendering in accordance with the invention is basedon the gradient calendering principle known per se, in which thesurfaces of the web are densified and the middle part of the web remainsbulky. When a web with a suitable moisture content is brought underpressure in a pre-calendering nip into contact with a hot backupsurface, the surface temperature of the web can be raised above theglass transition temperature of fibres, which is dependent on themoisture of the fibres. The intention is to bring the surface of the webinto a state in which the plastic deformation of the fibres ispermanent. In that connection, the fibres in the surface of the web donot return to their original round shape and position in the treatmentstages which take place subsequent to pre-calendering and increase themoisture content of the surface of the web. The relaxation of fibres,i.e. return to the original form, causes non-desirable re-roughening ofthe web surface.

By the thermal effect of the pre-calender thermo roll, the surface ofthe web lying against the thermo roll is closed. The pre-calender alsoimproves the smoothness of the surfaces of the web and attaches loosefibres or other stock components to the surface of the web. Thepre-calender also makes it possible to correct the two-sidednessassociated with the smoothness and oil absorbency of the web. This canbe done in a pre-calender provided with two nips by regulation of themoisture content of the web and/or by regulation of the temperatures ofthe thermo rolls. In each pre-calendering nip, the calendering effect isstrongest on the surface of the web lying against the thermo roll. Inthe first pre-calendering nip, one surface of the web will be againstthe thermo roll and, in the second pre-calendering nip, the oppositesurface of the web will be against the thermo roll. Since the web driesin the first pre-calendering nip, the calendering effect of the secondpre-calendering nip is smaller than that of the first one if the sametemperatures are used in the thermo rolls. The calendering effect of thesecond pre-calendering nip can be enhanced by moisturizing the webbetween the pre-calendering nips. The calendering effect of the secondpre-calendering nip can also be enhanced by using a higher temperaturein the second pre-calendering nip than in the first pre-calendering nip.Of course, the magnitude of the linear load used in each pre-calenderingnip also has an effect on the calendering efficiency of said nip.

Some bulk is lost in the base paper in the reinforced pre-calendering inaccordance with the invention but, surprisingly, the bulk of the endproduct remains on at least the same level as that of the LWCR printingpaper manufactured by the blade coating method in accordance with thestate of the art.

One explanation for this phenomenon is that, because of reinforcedpre-calendering, end calendering can be made lighter. Thus, the loss ofbulk in end calendering remains smaller than in the arrangements of thestate of the art.

Another explanation for this phenomenon is probably found in the pulpused in the manufacture of paper. The film coating method or thenon-contact coating method used in the arrangement in accordance withthe invention makes it possible to drop the proportion of chemical pulpbelow 30%. The film coating method and the non-contact coating method donot require the same strength from the base paper as the blade coatingmethod. The proportion of mechanical pulp can in turn be raised to atleast 70%. In addition, recycled fibres can be used in the pulp, inwhich connection the chemical fibres contained in the recycled fibresare included in the above-mentioned proportion of chemical pulp and themechanical fibres contained in the recycled fibres are included in theabove-mentioned proportion of mechanical pulp. The ability of mechanicalpulp fibres to recover their original shape after compression takingplace during calendering is considerably better than that of chemicalpulp fibres and thus a larger portion of mechanical pulp helps the bulkof the middle part of the web to be preserved, although the surfaces ofthe web are densified.

Increasing the proportion of mechanical pulp also improves the formationof the web, i.e. small-scale variation of the basis weight in the basepaper decreases. Mechanical pulp, which comprises shorter fibres thanchemical pulp, forms less flocs in the forming section, whereby the webbecomes more even. This also leads to the fact that the porosity of thesurface layers of the web is already reduced in the forming section,with the result that the surfaces of the web become dense.

The light scattering coefficient of the base paper is also improved whenthe proportion of mechanical pulp is increased because the lightscattering coefficient of mechanical pulp is 60 and the light scatteringcoefficient of chemical pulp is 25. As a result of this, the opacity ofthe paper produced as the end product is improved.

A reduction in the proportion of chemical pulp also leads to savings incosts because chemical pulp is generally more expensive than mechanicalpulp. The use of recycled fibres also makes the pulp less expensive ascompared with the traditional pulp used in LWCR printing paper.

A higher than normal moisture content of the web when entering thepre-calender can be made use of in the pre-dryer section preceding thepre-calender. The web simply need not be dried as much in the pre-dryersection, which means that the energy demand of the pre-dryer section isreduced. Pre-calendering also reduces the need for end calendering. Endcalendering can be accomplished with a smaller number of nips or withlower linear loads. When the two-sidedness possibly found in the web inPPS roughness and Cobb-Unger oil absorption is corrected in thepre-calender, end calendering is facilitated.

By the method in accordance with the invention it is possible tomanufacture both LWCO and LWCR printing paper. The greatest advantage isachieved in the manufacture of LWCR printing paper because thestate-of-the-art blade coating method can be replaced with a moreefficient and less expensive film coating method or non-contact coatingmethod.

In the following, components suitable for the paper machine according tothe invention are described with reference to arrangements known per seand disclosed in patent literature or other publications. With thesereferences, said publications are incorporated in this application.

The short circulation process arrangement can be accomplished in themanner described in WO publication 99/64668 and marketed under thetrademark OptiFeed™. The mixing of component stocks takes placeimmediately after the proportioning chests of the component stocks in aclosed mixing volume, after which the stock is passed in a closed volumeto a headbox. By minimizing the volume of the short circulation and byusing an abundance of automation, the time taken by a grade change canbe shortened from one hour to a few tens of seconds. From the point ofview of the invention, it is also possible to use a traditional stockfeed arrangement that is based on a proportioning chest, a machinechest, and a wire pit. Fillers, fines and additives can be fed into thestock before the headbox or only in the headbox. Here, it is alsopossible to use a short circulation and/or a headbox that allowslayering of additives and/or fillers and/or fines. One stock feedarrangement of this kind is disclosed in EP patent 651 092.

As the headbox it is possible to use a single- or multilayer headbox.One multilayer headbox is described in PCT patent application F197/00713and in the paper by M. Odell: Multilayering, Method or Madness?, XIValmet Paper Technology Days 1998. The web can be provided with desiredlayer structures by means of a multilayer headbox by feeding the stockin layers between wires. The single- or multilayer headbox can be adilution headbox, which is marketed under the trademark OptiFlo™. Inthis headbox, the basis weight profile can be regulated by consistencyregulation and in it it is possible to affect the fibre orientation byadjusting the profile. A dilution headbox allows a uniform profile to beimparted to the web both in the machine direction and in the crossdirection.

From the viewpoint of the invention, it is advantageous that the formingsection is based on a gap former, for instance, on a roll gap former, ablade-shoe gap former or a roll blade-shoe gap former. The gap former ispreferably provided with loading dewatering elements. In the gap former,a slice jet produced by the headbox is passed between two wires and mostof the water is removed between said wires in two directions, therebyproducing a symmetric web. U.S. Pat. No. 5,798,024 discloses oneadvantageous roll blade-shoe gap former applied in the invention. Thepaper by L. Verkasalo: Efficient forming at High Speeds, XI Valmet PaperTechnology Days 1998 describes one advantageous gap former sold underthe trademark OptiFormer™. The fibre and filler distribution in thethickness direction of the web can be controlled to some extent byplacement and vacuums of the dewatering elements of the gap former.Fillers are often accumulated on the surfaces of the web in thedewatering stages. A gap former allows higher speeds than those allowedby other types of formers and it provides good formation for the webthat is being formed. The dewatering blades fitted immediately after aforming roll reduce the layer thickness of the web before loadingblades. This in turn has a positive effect on the formation of the webthat is being formed. The formation of the web is advantageously below3.0 g/m². The blades can be loaded with a relatively high force becausethe internal bond strengths of the web that is being formed are not ofhigh significance when coated paper is manufactured. In this kind of gapformer, the surface of the web can be closed, so that the penetration ofthe coating agent into the web is reduced. The good formation, closedsurfaces, good symmetry, uniform profile and sufficient smoothnessattained by the gap former create good opportunities for the furthertreatment of the web.

The press section must have a closed and supported draw through theentire press section in order that it might be run at high speeds,typically over 1,800 m/min. The solids content of the web shall beraised in the press section to a value of over 45%, which is in itselfpossible in a press section provided with both roll nips and extendednips. A high solids content is required in order that the tensile stressdirected at the web between the press section and the dryer section maybe minimized. The speed difference of the web between the press sectionand the dryer section is advantageously below 2% because the porosity ofthe web increases with higher differences of draw. Good bulk, a highsolids content and a web that is as symmetric as possible are achievedby using a press section provided with one or more double-feltedextended-nip presses. WO publication 99/60202 discloses in oneembodiment a press section provided with two separate double-feltedextended nips. A press section is marketed under the trademarkOptiPress™ with one of its embodiments comprising two separate extendednips which are both double-felted. This kind of press section providessymmetric dewatering and a web which is symmetric in its surfaceproperties. One of the felts can also be replaced with a transfer beltthat does not receive water and transfers the web well. The smoothnessof the web and its two-sidedness as well as the absorbency of the webcan be controlled by means of transfer belts and press felts.

In the pre-dryer section it is possible to use cylinder drying and/orblowing drying, for example, impingement drying and/or through drying.The upstream end of the pre-dryer section in particular is important inorder that the speed difference of the web between the press section andthe pre-dryer section should remain as low as possible. Efficient dryingis achieved and the speed difference is minimized in the blowing dryerportion situated at the beginning of the pre-dryer section. The blowingdrying also speeds up grade change because the regulation of thetemperature of the blowing drying units is quick. The pre-dryer sectioncan start from a planar dryer portion that applies blowing drying, whichis followed by a cylinder drying portion. In the cylinder drying groupsit is also possible to use suction cylinders which are placed in thebasement spaces, which have a large diameter and in connection withwhich impingement units are placed for drying the web running on theouter surface of the suction cylinder. Impingement drying also enablesmore efficient control of the moisture profile as compared with merecylinder drying PCT patent application F198/00945 discloses a dryersection that applies impingement and which is marketed under thetrademark OptiDry™. The pre-dryer section can also be provided withsteaming or moisturizing devices known in themselves to allow curl ofthe paper web to be controlled and regulated.

After the pre-dryer section the web is passed to a pre-calender, whichcan be formed of a soft calender or an extended nip calender. Theextended nip calender can be a shoe calender or a belt calender. In ashoe calender, the extended nip is formed between a shoe roll and a hardsurface thermo roll. The shoe roll comprises a stationary supportstructure and an elastic belt shell disposed rotatably around it. Thebelt shell is loaded against the thermo roll by means of a loading shoewhich is supported on the support structure of the shoe roll and whichforms an extended nip with the thermo roll. The belt calender can beformed by a thermo roll, a belt loop and a backup roll, which can beeither a hard surface or a soft surface roll. The belt runs over thebackup roll and guide/tension rolls. In this kind of belt calender, theextended nip is formed between the belt and the hot thermo roll, inwhich connection, for example, a metal belt can be used. One such beltcalender is disclosed in U.S. Pat. No. 5,483,873. As the pre-calender itis also possible to use a one-nip calender if both calendering backupsufiaces are heatable. FI patent application 971342 discloses one beltcalender application in which the web runs in a nip between two elasticsurface rolls between metal belts running around said rolls. If bothmetal belts are heated, a one-nip calender is provided in which bothsurfaces of the web can be subjected to the treatment of a hotcalendering backup surface. The pre-calender is, however, advantageouslya two-nip or multi-nip calender in which both surfaces of the web can besubjected to the treatment of a thermo roll. One advantageous calendersuitable for a pre-calender is the soft calender which is marketed underthe trademark OptiSoft™ in which a nip is formed between a roll providedwith an elastic cover and a hard surface thermo roll. One OptiSoft™calender application is disclosed in FI patent application 992214.Another advantageous calender suitable for a pre-calender is the shoecalender which is marketed under the trademark OptiDwell™ and of whichone application is disclosed in U.S. Pat. No. 6,158,333.

The pre-calender is followed by coating of the web, by which the surfacestructure of the base paper is evened out. Coating can be performed byapplying a film coating method or a non-contact coating method. Thenon-contact coating methods include, among other things, a spray coatingmethod, a curtain coating method and a dry coating method. FI patent97247 discloses a spray coating method marketed under the trademarkOptiSpray™. FI patent application 991863 describes a curtain coatingmethod. It is common to these coating methods that the coating layerbecomes substantially equally thick. The coating layer conforms to thesurface profile of the base paper, with the result that an uneven websurface causes an uneven coating surface. Coating is performedadvantageously in a compact coating station, in which both surfaces ofthe web can be coated simultaneously. A film coating method is verysuitable for such coating. The closed and smooth surface of the webachieved by pre-calendering provides good conditions for coating. Inthat connection, it is preferable to use a profiling coating device,which can be controlled automatically based on profile measurements.This ensures a good cross-direction profile for the coated web and auniform quality for the coated paper. As an applicator device it ispossible to use a two-side film coating device which is marketed underthe trademark OptiSizer™ and in which both surfaces of the web can betreated simultaneously with a starch or pigment suspension. When needed,very light coating can be performed by this kind of applicator device,in which connection the basis weight of coating is at least about 2g/m²/page. One coating device of this type is described in FI patent81734.

The coating station is followed by an after-dryer section which startswith a dryer portion applying contact-free drying. The contact-freedrying is followed by a short cylinder group, by which the stabilizingof the running of the web, the draw and tension of the web are primarilyaffected at the same time as drying is continued. The cylinder groupadvantageously applies single-wire draw. In connection with contact-freedrying, it is possible to apply the drying which is marketed under thetrademark TurnDry™ and in which the paper web is dried, supported in acontact-free manner and turned by the same device, for example, by acombination of a turning device and an airborne web-dryer. This enablesa quick grade change and, at the same time, stable running of the web isassured. One such drying method is described in FI patent 98944.

After that, there is on-line end calendering, the aim of which isprimarily to improve the gloss of the coated web because the requiredsmoothness of the web has already been achieved in the pre-calender. Thetwo-sidedness associated with the roughness of the web has also beencorrected on the pre-calender, so that it is not any more necessary tomake so much correction in the end calender. As a result of this, endcalendering can be made lighter. End calendering can be performed by asoft calender or a multi-nip calender. As a soft calender it is possibleto use a calender marketed under the trademark OptiSoft™ having a nipwhich is formed between a roll provided with an elastic cover and a hardsurface thermo roll. One OptiSoft™ calender application is disclosed inFI patent application 992214. The calender described in FI patent 96334and marketed under the trademark OptiLoad™ can be used as a multi-nipcalender, the rolls of the roll stack in said calender being relievedsuch that there is the same linear load in each nip. By the multi-nipcalender is here meant a calender which comprises at least three rollsin contact with one another such that two nips are formed between therolls. The roll stack of the multi-nip calender can be located in avertical plane or the roll stack can form an angle with a verticalplane. The multi-nip calender can also be formed of several separateroll stacks mounted on the same frame or on different frames, so thateach roll stack forms, in a way, a multi-nip calender of its own. A softcalender having 2-4 nips and a multi-nip calender having 4-7 nipsprovide the gloss and the smoothness required from the end product inthe arrangement in accordance with the invention. In that connection,the surface temperature of the thermo rolls in the soft calender is atleast 150° C., the linear load is in a range of 50-500 kN/m, and thesurface temperature of the thermo rolls in the multi-nip calender is atleast 120° C. and the linear load is in a range of 150-600 kN/m. Themoisture content of the web before end calendering is regulated to be ina range of 5-11%, advantageously in a range of 5-9%.

The paper machine ends in a reel-up, for example, a reel-up which ismarketed under the trademark OptiReel™ and of which one application isdescribed in FI patent 91383. This kind of reel-up allows the amount ofbottom broke to be minimized and a high-quality reel to be achieved, sothat further processing of the reel takes place without problems.

Appropriate automation and measurement devices are combined with themethod and the paper machine for manufacturing LWC printing paper coatedonce in accordance with the invention, for example, for the purpose ofdetermining and correcting longitudinal and cross-direction profiles ofthe web or for accomplishing a quick grade change. As a measurementdevice is used, for example, a cross beam having several sensors orscanners and, at the same time, it is possible to measuremachine-direction variation, for example, by scanning devices.

In summary, it may be stated that high-quality LWC printing paper whichis coated once can be produced efficiently by the paper machineaccording to the invention.

By using profiling devices, profile variations found in the web can becorrected in different parts of the manufacturing line. The basis weightcan be profiled by regulating the consistency of the headbox. In thepress section, a steam box can be used for increasing and profiling thesolids content. Blowing drying enables the drying to be profiled and inthe dryer section it is also possible to use a moisturizing device forprofiling the solids content. Surface size/coating amount can beprofiled by coaters of the OptiSizer™ type. In connection with theinvention, it is also possible to use arrangements suitable for controlof curl of a paper web, these arrangements being described in FI patentapplications 906216, 950434, 964830 and 972080.

In the following, the invention will be described with reference to thefigures in the appended drawings, to the details of which the inventionis, however, not meant to be exclusively confined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a forming section and a press section of a paper machine inaccordance with the invention.

FIG. 2 shows the upstream end of a pre-dryer section.

FIG. 3 shows the downstream end of the pre-dryer section.

FIG. 4 shows a pre-calender, a coating station and an after-dryersection.

FIG. 5 shows an end calender and a reel-up.

FIG. 6 shows the PPS-s10 roughness of base paper as a function of bulkfor an uncalendered base paper, a conventionally pre-calendered basepaper and a base paper subjected to reinforced pre-calendering.

FIG. 7 shows the Cobb-Unger oil absorption of base paper as a functionof bulk for an uncalendered base paper, a conventionally pre-calenderedbase paper and a base paper subjected to reinforced pre-calendering.

FIG. 8 shows the PPS-s10 roughness of coated paper as a function of bulkfor an LWCO printing paper which has been pre-calendered in accordancewith the state of the art and coated by a blade coating method and afilm coating method, and for an LWCR printing paper which has beencoated by a blade coating method.

FIG. 9 shows, as a function of bulk, the roughness of LWCO printingpaper which has been film-coated once and pre-calendered in accordancewith the state of the art and the roughness of an LWCO printing paperwhich has been film-coated once and subjected to reinforcedpre-calendering.

FIG. 10 shows, as a function of bulk, the roughness of an LWCR printingpaper which has been film coated once and pre-calendered in accordancewith the state of the art and the roughness of an LWCR printing paperwhich has been film coated once and subjected to reinforcedpre-calendering.

The paper machine shown in FIGS. 1-5 comprises, in the running directionof a web W, a headbox 100, a gap former 200, a press section 300, apre-dryer section 400, a pre-calender 500, a coating station 600, anafter-dryer section 700, an end calender 800, and a reel-up 900.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the upstream end of the paper machine, i.e. the headbox100, the gap former 200 and the press section 300. The headbox 100 isadvantageously a dilution headbox and it may also include layering offibres and/or fillers and/or fines and/or additives. The gap former 200comprises a first wire loop 201 and a second wire loop 202, betweenwhich loops a substantially vertical forming zone is formed. Stock isfed from the headbox 100 into a gap formed by the first 201 and thesecond 202 wire loop between a forming suction roll 203 and a breastroll 204. In the forming zone, a first dewatering unit 206 is arrangedinside the first wire loop 201 and a second dewatering unit 207comprising loading dewatering members is arranged inside the second wireloop 202. Water is removed from the web and the formation of the webthat is being formed is improved by means of the dewatering units 206,207. At the end of the forming zone, the running direction of the formedweb W is turned by means of the vacuum of a suction roll 205 placedinside the second wire loop 202, by the suction of which vacuum the webW is separated from the first wire 201 and caused to adhere to thesecond wire 202. After that, the web W is transferred on support of thesecond wire 202 to a pick-up point P, at which the web W is separatedfrom the second wire 202 by pick-up suction roll 303 and transferred onsupport of a first press felt 301, i.e. a pick-up felt, to the presssection 300.

In the press section 300, the web W is passed between the first upperpress felt 301 and a second lower press felt 302, where the web W runsto a first press nip NP1. The first press nip NP1 is an extended nipformed by a lower shoe roll 306 provided with a loading shoe and a beltshell and by an upper recessed surface counter roll 305. After the firstpress nip NP1, the web W is separated from the first press felt 301 at afirst transfer point S1 by means of the vacuum of a first transfersuction roll 304 situated inside the second press felt loop 302 andcaused to adhere to the second press felt 302. After that, the web W ispassed on support of the second press felt 302 to a second transferpoint S2, at which the web is separated from the second press felt 302by means of the vacuum of a second transfer suction roll 313 situatedinside a third press felt loop 311 and caused to adhere to the thirdpress felt 311. After that, the web W is passed on support of the thirdpress felt 311 to a second press nip NP2. The web W runs in the secondpress nip NP2 between the third upper press felt 311 and a fourth lowerpress felt 312. The second press nip NP2 is an extended nip, which isformed by an upper shoe roll 316 provided with a loading shoe and a beltshell and by a lower recessed surface counter roll 315. After the secondpress nip NP2, the web W is separated from the third press felt 311 andtransferred on support of the fourth press felt 312 to a third transferpoint S3, at which the web W is separated from the fourth press felt 312by means of the vacuum of a fourth transfer suction roll 410 situatedinside a drying wire loop 419 of a first drying group R1 in thepre-dryer section 400. Afer that, the web W is transferred on support ofsaid drying wire 419 to the pre-dryer section 400.

Here it is also possible to use a press section in which one of thepress felts 311, 312 of the second press nip NP2 is replaced with asubstantially non-water-receiving transfer belt. By the transfer belt itcan be ensured that, after the second press nip NP2, the web W followsthe transfer belt on whose surface the web W is passed to the pre-dryersection 400.

FIG. 2 shows the upstream end of the pre-dryer section 400 illustratingfirst three drying groups R1, R2, R3 that apply single-wire draw. Thefirst group R1 is a downwards open drying group R1 in which heateddrying cylinders 411, 412, 413 are above and reversing suction rolls414, 415 are below.

The web W is brought to the pre-dryer section 400 on support of thedrying wire 419 of the first drying group R1 aided by the vacuum of asuction box 416 disposed inside said drying wire loop 419. After that,the web W runs along a meandering path between the drying cylinders 411,412, 413 and the reversing suction rolls 414, 415. Above the firstreversing suction roll 414 there is a runnability component 417, whichensures the running of the web W in the portion between the firstreversing suction roll 414 and the upper drying cylinders 411, 412.

From the last drying cylinder 413 of the first drying group R1 the web Wis passed in a nip between said drying cylinder 413 and a drying wire429 of the second drying group R2 onto the drying wire 429 of the seconddrying group R2 and to a first reversing suction roll 424 of the seconddrying group R2. From said reversing suction roll 424 the web W ispassed to a first drying cylinder 421 of the second drying group R2 andtherefrom further to a large-diameter impingement and/or through dryingcylinder 420, preferably having a diameter of over 4 m, situated belowthe floor level of the paper machine hall. The impingement units of theimpingement drying cylinder 420 are designated by the reference numerals420 a and 420 b. From the impingement drying cylinder 420 the web W ispassed to a second drying cylinder 422 of the second drying group R2 andtherefrom further in a meandering fashion over a second reversingsuction roll 425 of the second drying group R2 to the last dryingcylinder 423 of the second drying group R2. Runnability components 426,427 are disposed on the reversing suction rolls 424, 425 of the seconddrying group R2. The impingement and/or through drying cylinder 420placed below the floor level of the paper machine hall provides a longdrying distance for the web in relation to the machine direction advanceof the web.

From the last drying cylinder 423 of the second drying group R2, the webW passes in a nip between said drying cylinder 423 and a drying wire 439of the third drying group R3 onto the drying wire 439 of the thirddrying group R3 and to a first reversing suction roll 434 of the thirddrying group R3. After that, the web W runs in a meandering fashionbetween drying cylinders 431, 432, 433 and reversing suction rolls 435,436 of the third drying group R3. A runnability component 437 isdisposed on the first reversing suction roll 434 of the third dryinggroup R3.

FIG. 3 shows the downstream end of the pre-dryer section 400illustrating latter four drying groups R4-R7. The fourth R4 and thesixth R6 drying group correspond in structure to the second drying groupR2. The fifth R5 and the seventh R7 drying group in turn correspond instructure to the third drying group R3. The pre-dryer section 400 thuscomprises in total seven drying groups R1-R7. The second R2, the fourthR4 and the sixth R6 drying group are provided with impingement cylinders420, 440, 460 situated below the floor level and with associatedimpingement units 420 a, 420 b; 440 a, 440B; 460 a, 460 b. The pre-dryersection 400 also includes the elements 441, 442, 443, 444, 445, 446, 447in the fourth drying group R4; the elements 451, 452, 453, 454, 455,456, 457, 459 in the fifth drying group R5; the elements 461, 462, 463,464, 465, 466, 467, and 469 in the sixth drying group R6; and theelements 471, 472, 473, 474, 475, 476, 477, and 479 in the seventhdrying group R7.

The run of the web W is closed and supported from the beginning of theforming section 200 to the end of the pre-dryer section 400.

FIG. 4 shows the pre-calender 500, the coating station 600 and theafter-dryer section 700 situated after the pre-dryer section 400.

From the last drying cylinder 473 of the last i.e. the seventh dryinggroup R7 in the pre-dryer section 400, the web W is passed as an opendraw via a first measurement device 490 to the pre-calender 500. Thecross profile of the web W is measured in the first measurement device490 in order that the variations found in it might be taken into accountin pre-calendering. The variations in the cross profile of the web aresought to be equalized by performing profiled pre-calendering.

The pre-calender 500 may be a soft calender, a shoe calender or a beltcalender. The nip of the soft calender is formed between a thermo rollhaving a hard surface and a backup roll having an elastic surface. Thenip of the shoe calender is formed between a thermo roll having a hardsurface and a shoe roll serving as a backup roll. The nip of the beltcalender can be formed between a thermo roll having a hard surface and ametal belt running around a backup roll having an elastic surface orbetween two rolls having a elastic surface and metal belts runningaround the rolls. In the figure, the pre-calender 500 is a two-nip NE1,NE2 shoe calender, in which the first pre-calendering nip NE1 is formedby a lower first shoe roll 510 and an upper hard surface first thermoroll 511. The second pre-calendering nip NE2 is formed by an uppersecond shoe roll 520 and a lower hard surface second thermo roll 521.The upper surface of the web W is thus placed against the outer surfaceof the shell of the first thermo roll 511 in the first pre-calenderingnip NE1 and the lower surface of the web W is placed against the outersurface of the shell of the second thermo roll 521 in the secondpre-calendering nip NE2.

In connection with the pre-calender 500, there is also shown a firstmoisturizing device 512 of the web W, which is situated before the firstpre-calendering nip NE1 on the side of the thermo roll 511 of the firstpre-calendering nip NE1, and a second moisturizing device 522 of the webW, which is situated between the first pre-calendering nip NE1 and thesecond pre-calendering nip NE2 on the side of the thermo roll 521 of thesecond pre-calendering nip NE2. When needed, the moisture content of theweb W can be regulated by the moisturizing devices 512, 522 so as to besuitable separately for each pre-calendering nip NE1, NE2. As themoisturizing devices 512, 522 it is possible to use steam moisturizingdevices or water moisturizing devices known in themselves. The firstmoisturizing device 512 is necessary only in the cases in which the webW has been dried too dry in the pre-dryer section 400, in whichconnection the moisture content of the web W must be raised beforepre-calendering. This is, of course, not desirable but the web W shouldbe dried already in the pre-dryer section 400 to a correct solidscontent appropriate for pre-calendering. If there is no need to enhancethe calendering effect of the second pre-calendering nip NE2, the secondmoisturizing device 522 can also be omitted. Moisturizing can also bearranged such that a moisturizing device 512, 512 a is arranged on bothsides of the web W before the first pre-calendering nip NE1 of thepre-calender 500 and/or such that a moisturizing device 522, 522 a isarranged on both sides of the web W before the second pre-calenderingnip NE2 of the pre-calender 500.

The paper machine may have a pre-calender 500 which is two-sided, sothat the asymmetry of the porosity and roughness of the surfaces of theweb W is equalized in the pre-calender 500 by two-sided calendering, inwhich the more porous surface of the web is densified more than thedenser surface of the web by regulating the moisture content 512, 522 ofthe web and/or the temperature of the thermo rolls 511, 521 of the nipsNE1, NE2.

The moisturizing device 512, 512 a is arranged at a suitable distancebefore the nip of the pre-calender such that the time of action of waterbefore the nip is in a range of 0.05-0.5 s. The aim of an appropriatelyselected time of action is to cause both surfaces of the web to bemoistened while the middle part of the web remains substantiallynon-moistened. In a two-nip calender, in which the web runs only a shortdistance between the nips, the web can be guided, when needed, to anadditional loop between the nips in order that moisturizing shall have asufficiently long time of action. Another possibility is thatmoisturizing is performed only before the first nip of the pre-calender.In moisturizing, water is applied to the web in an amount of 1-4g/m²/side in order to achieve a desired moisture content. The initialmoisture of the web, the temperatures of the thermo rolls of thepre-calender and the linear loads of the pre-calender determine the needfor moisturizing.

After the pre-calender 500, the web W is passed via a second measurementdevice 590 to the coating station 600. The coating station 600 is acoating station 600 which applies roll application 611, 612 and is basedon film transfer and in which both surfaces of the web W aresurface-sized/pigmented simultaneously. This kind of coating station isvery efficient and it becomes relatively short in the machine direction.The second measurement device 590 measures the cross profile of the webW in order that the variations found in it might be taken into accountin coating. The variations in the cross profile of the web are sought tobe equalized by performing profiled coating.

After the coating station 600, the web W is passed to the after-dryersection 700. The after-dryer section 700 is mainly formed of a portion710 applying contact-free drying and of a short cylinder group 720applying single-wire draw. The portion 710 applying contact-free dryingcomprises an airborne web-dryer 711, a contact-free turning device 712for the web W, and an infrared drying unit 713. The cylinder group 720comprises a drying wire 729, heated drying cylinders 721, 722 and areversing suction roll 723 between them. Between the portion 710applying contact-free drying and the cylinder group 720, the web W ispassed to a third measurement device 790. The third measurement device790 measures the profile of the web W in order that the variations foundin it might be taken into account in end calendering. The variations inthe cross profile of the web are sought to be equalized by performingprofiled end calendering.

FIG. 5 shows the end calender 800 and the reel-up 900.

From the last drying cylinder 722 of the cylinder drying group 720 ofthe after-dryer section 700, the web W is passed to the end calender800, which is formed of a two-nip soft calender. A first calendering nipN1 is formed between a lower roll 810 having an elastic surface and anupper thermo roll 811 having a hard surface, and a second calenderingnip N2 is formed between an upper roll 820 having an elastic surface anda lower thermo roll 821 having a hard surface. As the end calender 800it is also possible to use a soft calender having 4 nips or a multi-nipcalender having 4-7 nips. In the end calender 800, the gloss of thesurfaces of the web W is mainly increased. From the last calendering nipN2 of the end calender 800, the web W is passed to the reel-up 900 inwhich machine reels 910 are made out of the web.

FIG. 6 shows the PPS-s10 roughness of base paper as a function of bulk.The filled squares represent the values measured from an uncalenderedbase paper, in which connection PPS-s 10 roughness is in a range of6.2-7.1 μm and bulk is in a range of 1.95-2.21 cm³/g. The unfilleddiamonds represent the values measured from a base paper pre-calenderedby a one-nip machine calender in accordance with the state of the art,in which connection PPS-s10 roughness is in a range of 4.0-5.6 μm andbulk is in a range of 1.7-1.9 cm³/g. The filled circles represent thevalues measured from a base paper subjected to reinforcedpre-calendering on a two-nip soft calender, in which connection PPS-s10roughness is in a range of 2.2-3.4 μm and bulk is in a range of1.22-1.52 cm³/g. The filled triangles represent the values measured froma base paper subjected to reinforced pre-calendering on a four-nip softcalender, in which connection PPS-s10 roughness is in a range of 2.1-2.8μm and bulk is in a range of 1.22-1.32 cm³/g. The unfilled squaresrepresent the values measured from a base paper subjected to reinforcedpre-calendering on a two-nip shoe calender, in which connection PPS-s10roughness is in a range of 2.6-3.0 μm and bulk is in a range of1.45-1.58 cm³/g. The figure shows that values of below 3.5 μm in PPS-s10toughness can be achieved with a base paper subjected to reinforcedpre-calendering but, at the same time, some bulk is lost as comparedwith traditional pre-calendering.

FIG. 7 shows the Cobb-Unger oil absorption of base paper as a functionof bulk. The filled squares represent the values measured from anuncalendered base paper, in which connection Cobb-Unger oil absorptionis in a range of 16-28 g/m² and bulk is in a range of 1.95-2.21 cm³/g.The unfilled diamonds represent the values measured from a base paperpre-calendered by a one-nip machine calender in accordance with thestate of the art, in which connection Cobb-Unger oil absorption is in arange of 13-26 g/m² and bulk is in a range of 1.7-1.9 cm³/g. The filledcircles represent the values measured from a base paper subjected toreinforced pre-calendering on a two-nip soft calender, in whichconnection Cobb-Unger oil absorption is in a range of 6.5-14.5 g/m² andbulk is in a range of 1.22-1.48 cm³/g. The filled triangles representthe values measured from a base paper subjected to reinforcedpre-calendering on a four-nip soft calender, in which connectionCobb-Unger oil absorption is in a range of 8-13.5 g/m² and bulk is in arange of 1.22-1.32 cm³/g. The unfilled squares represent the valuesmeasured from a base paper subjected to reinforced pre-calendering on atwo-nip shoe calender, in which connection Cobb-Unger oil absorption isin a range of 12-15 g/m² and bulk is in a range of 1.45-1.59 cm³/g. Thefigure shows that values of below 15 g/m² in Cobb-Unger oil absorptioncan be achieved with a base paper subjected to reinforcedpre-calendering but, at the same time, some bulk is lost as comparedwith traditional pre-calendering.

It is seen from FIGS. 6 and 7 that a sufficiently low PPS-s10 roughnessand a sufficiently low Cobb-Unger oil absorption are achieved by a basepaper which is subjected to reinforced pre-calendering in order that thebase paper may be coated by applying a film coating method or anon-contact coating method for the manufacture of LWCR paper. ThePPS-s10 roughness and Cobb-Unger oil absorption of the traditionallypre-calendered base paper require a blade coating method in order thatpaper meeting the quality requirements of LWCR might be manufactured.

FIG. 8 shows the PPS-s10 roughness of a printing paper coated once as afunction of bulk for LWC paper. The base paper has been manufactured bypre-calendering in accordance with the state of the art on a one-nipmachine calender, in which the temperature of the thermo roll was60-100° C. and the linear load was 10-60 kN/m. The filled diamondsrepresent the values measured from an LWCO paper coated by a filmcoating method or a non-contact coating method, in which connectionPPS-s10 roughness is in a range of 1.12-1.7 μm and bulk is in a range of0.815-0.93 cm³/g. The unfilled circles in turn represent the valuesmeasured from an LWCO paper coated by a blade coating method, in whichconnection PPS-s10 roughness is in a range of 0.9-1.57 μm and bulk is ina range of 0.81-1.0 cm³/g. The unfilled triangles represent the valuesmeasured from an LWCR paper coated by a blade coating method, in whichconnection PPS-s10 roughness is in a range of 0.6-0.92 μm and bulk is ina range of 0.77-0.92 cm³/g.

FIG. 9 shows, as a function of bulk, the roughness of LWCO printingpaper coated once by a film coating method. The unfilled diamondsrepresent the values measured from a printing paper pre-calendered by aone-nip machine calender in accordance with the state of the art andfilm-coated once, in which connection PPS-s10 roughness is in a range of1.7-1.75 μm and bulk is about 0.94 cm³/g. The filled circles representthe values measured from a printing paper subjected to reinforcedpre-calendering on a two-nip soft calender and film-coated once, inwhich connection PPS-s10 roughness is in a range of 1.21-1.39 μm andbulk is in a range of 0.9-0.95 cm³/g. The results show that the PPS-s10roughness of the LWCO printing paper manufactured in accordance with theinvention is lower and its bulk is almost as high as the correspondingvalues of the LWCO printing paper manufactured by traditionalpre-calendering.

FIG. 10 shows, as a function of bulk, the roughness of LWCR printingpaper coated once by a film coating method. The unfilled diamondsrepresent the values measured from an LWCR printing paper pre-calenderedby a one-nip machine calender in accordance with the state of the artand coated once by a blade coating method, in which connection PPS-s10roughness is in a range of 1.15-1.30 μm and bulk is in a range of0.93-1.02 cm³/g. The filled circles represent the values measured froman LWCR printing paper subjected to reinforced pre-calendering on atwo-nip soft calender and film-coated once, in which connection PPS-s 10roughness is in a range of 0.85-1.15 μm and bulk is in a range of0.875-1.02 cm³/g. The filled triangles represent the values measuredfrom an LWCR printing paper subjected to reinforced pre-calendering on afour-nip soft calender and film-coated once, in which connection PPS-s10roughness is in a range of 0.97-1.29 μm and bulk is in a range of0.92-1.00 cm³/g. The unfilled squares represent the values measured froman LWCR printing paper subjected to reinforced pre-calendering on atwo-nip shoe calender and film-coated once, in which connection PPS-s10roughness is in a range of 1.1-1.21 μm and bulk is in a range of0.92-0.97 cm³/g. The results show that the PPS-s10 roughness and bulk ofthe LWCR printing paper manufactured in accordance with the inventionare on the same level as the corresponding values of the LWCR printingpaper manufactured by traditional pre-calendering.

The results according to the invention shown in FIGS. 6-10 have beenobtained with a base paper that contained at least 70% mechanical pulp.In the pulp, 0-40% of recycled fibres was also used, in which connectionthe mechanical fibres contained in the recycled fibres were included inthe above-mentioned proportion of mechanical pulp. The proportion ofchemical pulp was 0-30%, which also included the chemical fibrescontained in the recycled fibres. The pre-calendering in accordance withthe state of the art was performed on a one-nip machine calender inwhich the temperature of the thermo roll was in a range of 60-100° C.and the linear load was in a range of 10-60 kN/m. In reinforcedpre-calendering, a soft calender with 2-4 nips and a shoe calender with2 nips were used. The temperature of the thermo rolls of thepre-calenders was in a range of 20-300° C. and the linear loads were ina range of 50-500 kN/m. The moisture content of the base paper beforethe pre-calender was 5-20% and the running speeds used were in a rangeof 1500-2200 m/min. As the end calender, a soft calender with 24 nipswas used, in which connection the temperature of the thermo rolls was200° C., and a multi-nip calender with 5-7 nips, in which connection thetemperature of the thermo rolls was 150° C.

The results obtained by different pre-calendering arrangements are notnecessarily quite directly comparable with one another. The largesttrial series were performed on a two-nip soft calender, in whichconnection it has been possible to optimize better the pre-calenderingand/or end calendering parameters affecting the end result.

The claims are presented in the following and the details of theinvention may vary within the inventive idea defined by said claims anddiffer from the disclosure given above by way of example only.

1-44. (cancelled)
 45. A method for the manufacture of LWC printing papercoated once, comprising the following successive steps: feeding stockfrom a headbox to a gap former, in which water is removed from a web intwo directions; passing the web to a press section which comprises atleast one extended nip press and in which water is removed from the webby pressing; passing the web to a pre-dryer section, in which the web isdried by applying at least cylinder drying; regulating the web to amoisture content of 5-20% before a first nip of a pre-calender, having anip or nips, the nip or nips of the pre-calender having a total length;passing the web to the pre-calender and calendering a first side and asecond side of the web in the pre-calender, the pre-calender having afirst side backup surface for the web first side and a second sidebackup surface for the web second side, each of the first side backupsurface and the second side backup surface being heated to at least 200°C., the pre-calender nip or nips total length being at least in a rangeof 15-600 mm at a nip load of 50-500 kN/m, in which the web ispre-calendered; passing the web to at least one coating station and oneafter-dryer section, in which the web is coated on both sides by using afilm coating method or a noncontact coating method; drying the web atleast by contact-free drying; passing the web which is now LWC printingpaper coated once, to an end calender, in which the web is calendered;and passing the LWC printing paper coated once web, to a reel-up, inwhich the web is reeled into machine reels.
 46. The method of claim 45wherein each of the first side backup surface and the second side backupsurface has a surface temperature of at least 250° C.
 47. The method ofclaim 45 wherein the linear load of each nip is in a range of 100-400kN/m.
 48. The method of claim 45 wherein moisture content of the web isregulated to a range of 6-15% before the first nip.
 49. The method ofclaim 45 wherein the pre-calender nip or nips total length is in a rangeof 30-600 mm.
 50. The method of claim 45 wherein the web isprecalendered in at least two soft nip calenders, each soft nip calenderbeing formed between a thermo roll forming the backup surface and anelastic surface roll such that one of said thermo rolls is situated inthe nips alternately on the first side and the second side of the web.51. The method of claim 45 wherein the web is precalendered in at leasta first shoe calender forming the first nip between a first shoe rolland a hard surface thermo roll forming the first side backup surfacewhich engages the first side of the web and a second shoe calenderforming a second nip between a second shoe roll and a hard surfacethermo roll forming the second side backup surface which engages thesecond side of the web.
 52. The method of claim 45 wherein the firstside of the web is pressed against a thermo roll in the first nip, andis moisturized before the first nip, whereby the effect of thetemperature of the thermo roll of the pre-calender on the first side ofthe web is enhanced.
 53. The method of claim 45 wherein the second sideof the web is pressed against a second thermo roll in a second nip, andis moisturized between the first and the second nip, whereby the effectof the temperature of the thermo roll of the second nip on the secondside of the web is enhanced.
 54. The method of claim 50 whereinasymmetry of porosity and roughness of the first side and the secondside of the web is equalized in the pre-calender by two-sidedcalendering, in which one of said first side and said second side is amore porous surface of the web and is densified more than the other ofsaid first side and second side, by regulating the moisture content ofthe web or the temperature of the thermo rolls of the nips.
 55. Themethod of claim 45 wherein the precalender includes a belt calenderhaving a nip which is formed between a thermo roll and a metal beltrunning over a roll having an elastic surface, in which connection thesurface temperature of the thermo roll and the metal belt is at least200° C., the length of the nip is in a range of 15-600 mm, the linearload of the nip is in a range of 50-500 kN/m, and the moisture contentof the web before the pre-calender is in a range of 5-20%.
 56. Themethod of claim 55, wherein both sides of the web are moisturized beforethe pre-calender, whereby the effect of the temperature of the thermoroll and the hot metal belt of the nip on the web is enhanced.
 57. Themethod of claim 55 wherein a dilution headbox is used as the headbox.58. The method of claim 57 wherein the headbox used is a multilayerdilution headbox in which fibres. additives, or fillers are layered. 59.The method of claim 45 wherein a gap former provided with a forming rolland dewatering units is used as the gap former.
 60. The method of claim45 wherein the press section is a press section which is provided withfirst successive extended press nip and a second successive extendedpress nip, and wherein the first and second extended press nips areformed between a shoe roll provided with an elastic belt shell and arecessed surface counter roll, wherein the counter roll is in the firstpress nip on the first side of the web and in the second press nip onthe second side of the web.
 61. The method of claim 60 wherein the webruns in first and second press nips between two press felts.
 62. Themethod of claim 45 wherein the web is dried in the pre-dryer section bycylinder drying groups which apply single wire draw and by impingementdrying units which are disposed in connection with the cylinder dryinggroups.
 63. The method of claim 45 wherein the web is passed after thepre-dryer section and before the pre-calender to a first measurementdevice, in which a profile of the web is measured.
 64. The method ofclaim 45 wherein the web is passed after the pre-calender and before thecoating station to a second measurement device, in which the profile ofthe web is measured.
 65. The method of claim 45 wherein the web ispassed in the after-dryer section after a portion applying contact-freedrying and before a portion applying cylinder drying to a thirdmeasurement device, in which the profile of the web is measured.
 66. Themethod of claim 45 wherein profiling of the properties of the paper webis controlled based on profile measurements of the paper web.
 67. Themethod of claim 45 wherein in the pre-dryer section, the drying of theweb is profiled by impingement units associated with impingementcylinders.
 68. The method of claim 45 wherein the web is calendered inan end calender which is formed of a soft calender having 2-4 nips, thesoft calender having 1-2 thermo rolls having a surface temperature of atleast 150° C., the linear load of the end calender nips being in a rangeof 50-500 kN/m and the moisture content of the web before a first nip ofthe end calender is in a range of 5-11%.
 69. The method of claim 45wherein the web is calendered in an end calender which is formed of amulti nip calender having 4-7 nips, and having at least two thermorolls, the thermo rolls having surface temperatures of at least 120° C.,the linear load of the end calender nips being in a range of 150-600kN/m and the moisture content of the web before a first nip of the endcalender is in a range of 5-11%.
 70. A paper machine for the manufactureof LWC printing paper coated once, comprising arranged in the runningdirection of a web: a headbox from which stock is fed to a former; a gapformer arranged to remove water from the web in two directions, thepaper web being formed in the gap former, and the web having a firstside and a second side, the web being comprised of at least 70%mechanical pulp; a press section having at least one extended nip press,through which the web is passing; a pre-dryer section in which the webis dried by applying at least cylinder drying; a pre-calender having atleast one nip having a total length of the nip or nips in a range of15-600 mm, and a linear load of the pre-calendering nip or nips is in arange of 50-500 kN/m and two heatable backing backup surfaces having atemperature of least 200° C. which are engage with the first side andthe second side of the web and in which the web is precalendered, theweb positioned immediately before the at least one nip having a moisturecontent of 5-20%; a coating station having a supply of coating incontact with the first side of the web forming a film coating coater ora noncontact coater, and the coating station further having a supply ofcoating in contact with the second side of the web forming a filmcoating coater or a noncontact coater; a subsequent after-dryer sectionhaving a noncontact dryer through which the web passes; an end calenderin which the web is calendered; and a reel-up in which the web isreeled.
 71. The paper machine of claim 70, wherein the pre-calender isformed of a soft calender in which there are at least two nips which areeach formed by an elastic surface roll and a hard surface thermo roll,the hard surface thermo rolls forming the two heatable backing backupsurfaces.
 72. The paper machine of claim 70, wherein the pre-calender isformed of a shoe calender in which there are at least two nips which areeach formed by a shoe roll and a hard surface thermo roll, the hardsurface thermo rolls having surfaces which form the two heatable backingbackup surfaces.
 73. The paper machine of claim 70 wherein thepre-calender has at least a first nip and a second nip, and wherein thepaper machine comprises a first moisturizing device situated, in therunning direction of the web, before the first nip of the pre-calenderfor moisturizing at least that side of the web which will be against thethermo roll in the first nip, whereby the effect of the temperature ofthe thermo roll of the first nip on that side of the web is enhanced.74. The paper machine of claim 70 wherein the pre-calender has at leasta first nip and a second nip, and wherein the paper machine comprises asecond moisturizing device placed in connection with a run of the webbetween the first and the second nip of the pre-calender formoisturizing at least that side of the web which will be against thethermo roll in the second nip, whereby the effect of the temperature ofthe thermo roll of the second nip on that side of the web is enhanced.75. The paper machine of claim 70, wherein the pre-calender is formed ofa belt calender having a nip which is formed between a thermo roll and ametal belt running over a backup roll having an elastic surface, andwherein the surface temperature of the thermo roll and the metal belt isat least 200° C.
 76. The paper machine of claim 70, wherein the papermachine comprises moisturizing devices which are situated, in therunning direction of the web, before the pre-calender and by which thefirst and second sides of the web are moisturized, whereby the effect ofthe temperature of the two heatable backing backup surfaces on the webin the nip is enhanced.
 77. The paper machine of claim 70, wherein theheadbox is a dilution headbox.
 78. The paper machine of claim 70,wherein the headbox is a multilayer headbox.
 79. The paper machine ofclaim 70, wherein the gap former is a gap former provided with a formingsuction roll and with at least one blade loading unit.
 80. The papermachine of claim 70 wherein the press section is a press sectionprovided with two extended press nips, the extended press nips beingformed between a shoe roll, which is provided with a belt shell and aloading shoe, and a recessed surface counter roll.
 81. The paper machineof claim 80, wherein each extended press nip of the press section isdouble-felted.
 82. The paper machine of claim 70, wherein the pre-dryersection comprises cylinder drying groups which apply single-wire drawand to which impingement drying cylinders provided with impingementunits are connected.
 83. The paper machine of claim 70, wherein thecoating station is a film coating station applying two-side rollapplication.
 84. The paper machine of claim 70, wherein the noncontactdryer is followed by a one cylinder drying group.
 85. The paper machineof claim 70, further comprising a first measurement device which isplaced on a run of the web between the pre-dryer section and thepre-calender and in which the profile of the web is measured.
 86. Thepaper machine of claim 70, further comprising a second measurementdevice which is placed on a run of the web between the pre-calender andthe coating station and in which the profile of the web is measured. 87.The paper machine of claim 70, further comprising a third measurementdevice which is placed on a run of the web between the portion applyingcontact-free drying and the portion applying cylinder drying in theafter-dryer section and in which the profile of the web is measured. 88.The paper machine of claim 70, wherein the end calender is a softcalender having 2-4 nips.
 89. The paper machine of claim 70, wherein theend calender is a multi-nip calender having 4-7 nips.
 90. A base paperintended for LWC printing paper which is coated once, wherein the basepaper comprises at least 70% mechanical pulp, and that the PPSs10roughness of the base paper before coating is below 3.5 μm, that theCobb-Unger oil absorption of the base paper before coating is below 15g/m², and that the bulk of the base paper before coating is over 1.0cm³/g.
 91. A method for the manufacture of LWC printing paper coatedonce, comprising the following successive steps: feeding stock from aheadbox to a gap former, in which water is removed from a web which isbeing formed in two directions to form a web comprised of at least 70%mechanical pulp; passing the web to a press section which comprises atleast one extended nip press and in which water is removed from the webby pressing; passing the web to a pre-dryer section, in which the web isdried by applying at least cylinder drying; regulating the web to amoisture content of 5-20% before a first nip of a pre-calender, having anip or nips, the nip or nips of the pre-calender having a total length;passing the web to the precalender and calendering a first side and asecond side of the web, the precalender having a first side backupsurface for the web first side and a second side backup surface for theweb second side, each of the first and second backup surfaces heated toat least 200° C., the pre-calender nip or nips having a total length ina range of 15-600 mm at a nip load of 50-500 kN/m, in which the web isprecalendered to form a base web from the formed web having a PPSs10roughness below 3.5 μm, a Cobb-Unger oil absorption below 15 g/m², and abulk of over 1.0 cm³/g; passing the web to at least one coating stationand one after-dryer section, in which the web is coated on both sides byusing a film coating method or a non-contact coating method; drying theweb at least by contact-free drying; passing the web which is now LWCprinting paper coated once, to an end calender, in which the web iscalendered; and passing the LWC printing paper coated once web, to areel-up, in which the web is reeled into machine reels.
 92. The methodof claim 91 wherein the web is pre-calendered to form a base web fromthe formed web having a PPSs10 roughness below 3.0 μm, a Cobb-Unger oilabsorption below 12 g/m², and a bulk of over 1.2 cm³/g.